Whenever a damaging spell or ability hits the caster, the cast time will be set back by a given amount, slowing the casting. In general any melee attack (be it a normal or a special attack) or direct damage spell will cause interruption. Damage over Time spells and most Channelled spells will not cause interruption. The only channeled spell which does is Arcane Missiles.

When casting a spell: The first and second hit will add .5 secs each to the cast time. All hits after the second will have no effect.

When channelling a spell: The first and second hit reduce current duration by 25% of total duration each. All hits after the second will have no effect.

Completely absorbed damage does not cause interruption; however, as soon as a part of the damage is not absorbed, it does.

The spell with the highest interruption frequency is Arcane Missiles; it causes one interruption per second, which can be quite devastating to caster classes.

Interruption caused by counterspell effect abilities, for example Counterspell and Pummel, will cause your cast to be totally interrupted. The spell will not take effect. This interruption will change the color of your casting time bar to red and say "Interrupted". Furthermore it prevents any spell from the same school (not tree) as the interrupted one to be cast for the duration of the counterspell effect.

Counterspell effects provide no visual debuff and cannot be dispelled. However, there is a special resistance against it, which can be raised via talents or items. It is not displayed in the character information page.

Instant cast spells cannot be interrupted by these effects, as they have no cast time and as such can not be affected during casting. Any normally cast spell or channeled spell can be interrupted, though.